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With modern large scale spectroscopic surveys, such as the SDSS and LSS-GAC, Galactic astronomy has entered the era of millions of stellar spectra. Taking advantage of the huge spectroscopic database, we propose to use a “standard pair" technique to a) Estimate multi-band extinction towards sightlines of millions of stars; b) Detect and measure the diffuse interstellar bands in hundreds of thousands SDSS and LAMOST low-resolution spectra; c) Search for extremely faint emission line nebulae in the Galaxy; and d) Perform photometric calibration for wide field imaging surveys. In this contribution, we present some results of applying this technique to the SDSS data, and report preliminary results from the LAMOST data. [ABSTRACT FROM PUBLISHER]

We carry out plasma diagnostic analyses for 123 planetary nebulae (PNe) and 42 H ii regions using the N ii and O ii optical recombination lines (ORLs). New effective recombination coefficients for the N ii and O ii optical recombination spectra are used. These data were calculated under the intermediate coupling scheme for a number of electron temperature (Te) and density (Ne) cases. We use a new method to determine the Te’s and Ne’s for the nebular sample, combining the ORLs with the most reliable measurements for each ion and the predicted intensities that are based on the new atomic data. Uncertainties of the derived Te and Ne are estimated for each object.The diagnostic results from heavy element ORLs show reasonable agreement with previous calculations in the literature. We compare the electron temperatures derived from the N ii and O ii ORLs, Te(ORLs), and those from the collisionally excited lines (CELs), Te(CELs), as well as the hydrogen Balmer jump, Te(H i BJ), especially for the PNe with large abundance discrepancies. Temperatures from the He i recombination lines, Te(He i), are also used for comparison if available. For all the objects included in our sample, Te(ORLs) are lower than Te(H i BJ), which are in turn systematically lower than Te(CELs). PNe with Te(He i) available show the relation Te(ORLs) ≤ Te(He i) ≤ Te(H i BJ) ≤ Te(CELs), which is consistent with predictions from the bi-abundance nebular model postulated by Liu et al. [ABSTRACT FROM AUTHOR]

A thorough critical literature survey has been carried out for reliable measurements of oxygen and neon abundances of planetary nebulae (PNe) and H ii regions. By contrasting the results of PNe and of H ii regions, we aim to address the issues of the evolution of oxygen and neon in the interstellar medium (ISM) and in the late evolutionary phases of low- and intermediate-mass stars (LIMS), as well as the currently hotly disputed solar Ne/O abundance ratio. Through the comparisons, we find that neon abundance and Ne/O ratio increase with increasing oxygen abundance in both types of nebulae, with positive correlation coefficients larger than 0.75. The correlations suggest different enrichment mechanisms for oxygen and neon in the ISM, in the sense that the growth of neon is delayed compared to oxygen. The differences of abundances between PNe and H ii regions are mainly attributed to the results of nucleosynthesis and dredge-up processes that occurred in the progenitor stars of PNe. We find that both these α-elements are significantly enriched at low metallicity (initial oxygen abundance ≲8.0) but not at metallicity higher than the Small Magellanic Cloud (SMC). The fact that Ne/O ratios measured in PNe are almost the same as those in H ii regions, regardless of the metallicity, suggest a very similar production mechanism of neon and oxygen in intermediate-mass stars (IMS) of low initial metallicities and in more massive stars, a conjecture that requires verification by further theoretical studies. This result also strongly suggests that both the solar neon abundance and the Ne/O ratio should be revised upwards by ∼0.22 dex from the Asplund, Grevesse & Sauval values or by ∼0.14 dex from the Grevesse & Sauval values. [ABSTRACT FROM AUTHOR]

Results from the first dedicated study of Galactic planetary nebulae (PNe) by means of optical integral field spectroscopy with the Very Large Telescope Fibre Large Array Multi Element Spectrograph Argus integral field unit are presented. Three typical Galactic disc PNe have been mapped with the 11.5 × 7.2-arcsec2 Argus array: 2D spectral maps of the main shell of NGC 5882 and of large areas of NGC 6153 and NGC 7009 with 297 spatial pixels per target were obtained at subarcsec resolutions. A corresponding number of 297 spectra per target were obtained in the 396.4–507.8 nm range. Spatially resolved maps of emission lines and of nebular physical properties such as electron temperatures, densities and ionic abundances were produced. The abundances of helium and of doubly ionized carbon and oxygen, relative to hydrogen, were derived from optical recombination lines (ORLs), while those of O2+ were also derived from the classic collisionally excited lines (CELs). The occurrence of the abundance discrepancy problem, pertaining to oxygen, was investigated by mapping the ratio of ORL/CEL abundances for O2+[the abundance discrepancy factor (ADF)] across the face of the PNe. The ADF varies between targets and also with position within the targets, attaining values of ∼40 in the case of NGC 6153 and ∼30 in the case of NGC 7009. Correlations of the ADF with geometric distance from the central star and plasma surface brightness (for NGC 6153), as well as with [O iii] electron temperature, plasma ionization state and other physical properties of the targets are established. Very small values of the temperature fluctuation parameter in the plane of the sky, , are found in all cases. It is argued that these results provide further evidence for the existence in run-of-the-mill PNe of a distinct nebular component consisting of hydrogen-deficient, super-metal-rich plasma. The zones containing this posited component appear as undulations in the C ii and O ii ORL abundance diagnostics of about 2 spatial pixels across, and so any associated structures should have physical sizes of less than ∼1000 astronomical units. Regarding the origin of the inferred zones, we propose that circumstellar discs, Abell 30-type knots, or Helix-type cometary globules may be involved. Implications for emission-line studies of nebulae are discussed. [ABSTRACT FROM AUTHOR]

Deep long-slit optical spectrophotometric observations are presented for 25 Galactic bulge planetary nebulae (GBPNe) and six Galactic disc planetary nebulae (GDPNe). The spectra, combined with archival ultraviolet (UV) spectra obtained with the International Ultraviolet Explorer and infrared spectra obtained with the Infrared Space Observatory, have been used to carry out a detailed plasma diagnostic and element abundance analysis utilizing both collisional excited lines (CELs) and optical recombination lines (ORLs). Comparisons of plasma diagnostic and abundance analysis results obtained from CELs and ORLs reproduce many of the patterns previously found for GDPNe. In particular we show that the large discrepancies between electron temperatures ( Te values) derived from CELs and ORLs appear to be mainly caused by abnormally low values yielded by recombination lines and/or continua. Similarly, the large discrepancies between heavy element abundances deduced from ORLs and CELs are largely caused by abnormally high values obtained from ORLs, up to tens of solar in extreme cases. It appears that whatever mechanisms are causing the ubiquitous dichotomy between CELs and ORLs, their main effects are to enhance the emission of ORLs, but hardly affect that of CELs. It seems that heavy element abundances deduced from ORLs may not reflect the bulk composition of the nebula. Rather, our analysis suggests that ORLs of heavy element ions mainly originate from a previously unseen component of plasma of Te values of just a few hundred kelvins, which is too cool to excite any optical and UV CELs. We find that GBPNe are on the average 0.1–0.2 dex more metal-rich than GDPNe but have a mean C/O ratio that is approximately 0.2 dex lower. By comparing the observed relative abundances of heavy elements with recent theoretical predictions, we show that GBPNe probably evolved from a relatively metal-rich environment of initial , compared to an initial for GDPNe. In addition, we find that GBPNe tend to have more massive progenitor stars than GDPNe. GBPNe are found to have an average magnesium abundance about 0.13 dex higher than GDPNe. The latter have a mean magnesium abundance almost identical to the solar value. The enhancement of magnesium in GBPNe and the large [α/Fe] ratios of bulge giants suggest that the primary enrichment process in the bulge was Type II supernovae. PN observations yield a Ne/O abundance ratio much higher than the solar value, suggesting that the solar neon abundance may have been underestimated by 0.2 dex. [ABSTRACT FROM AUTHOR]

We present deep optical spectra of 23 galactic planetary nebulae, which are analysed in conjunction with archival infrared and ultraviolet spectra. We derive nebular electron temperatures based on standard collisionally excited line (CEL) diagnostics as well as the hydrogen Balmer jump and find that, as expected, the Balmer jump almost always yields a lower temperature than the [O iii] nebular-to-auroral line ratio. We also make use of the weak temperature dependence of helium and O ii recombination line ratios to further investigate the temperature structure of the sample nebulae. We find that, in almost every case, the derived temperatures follow the relation , which is the relation predicted by two-component nebular models in which one component is cold and hydrogen-deficient. Te(O ii) may be as low as a few hundred Kelvin, in line with the low temperatures found for the hydrogen-deficient knots of Abell 30 by Wesson, Liu and Barlow. Elemental abundances are derived for the sample nebulae from both CELs and optical recombination lines (ORLs). ORL abundances are higher than CEL abundances in every case, by factors ranging from 1.5 to 12. Five objects with O2+ abundance discrepancy factors greater than 5 are found. DdDm 1 and Vy 2–2 are both found to have a very large abundance discrepancy factor of 11.8. We consider the possible explanations for the observed discrepancies. From the observed differences between Te(O iii) and Te(BJ), we find that temperature fluctuations cannot resolve the abundance discrepancies in 22 of the 23 sample nebulae, implying some additional mechanism for enhancing ORL emission. In the one ambiguous case, the good agreement between abundances derived from temperature-insensitive infrared lines and temperature-sensitive optical lines also points away from temperature fluctuations being present. The observed recombination line temperatures, the large abundance discrepancies and the generally good agreement between infrared and optical CEL abundances all suggest instead the existence of a cold hydrogen-deficient component within the ‘normal’ nebular gas. The origin of this component is as yet unknown. [ABSTRACT FROM AUTHOR]

In Paper I, we presented spectrophotometric measurements of emission lines from the ultraviolet (UV) to the far-infrared for 12 Galactic planetary nebulae (PNe) and derived nebular thermal and density structures using a variety of plasma diagnostics. The measurements and plasma diagnostic results are used in the current paper to determine elemental abundances in these nebulae. Abundance analyses are carried out using both strong collisionally excited lines (CELs) and weak optical recombination lines (ORLs) from heavy element ions. Assuming electron temperatures and densities derived from HI recombination spectra (line and continuum), we are able to determine the ORL C abundance relative to hydrogen for all the PNe in our sample, N and O abundances for 11 of them and Ne abundances for nine of them. In all cases, ORL abundances are found to be systematically higher than the corresponding values deduced from CELs. In NGC 40, the discrepancy between the abundances derived from the two types of emission line reaches a factor of 17 for oxygen. For the other 10 PNe, the discrepancies for oxygen vary from 1.6 to 3.1. In general, collisionally excited infrared fine-structure lines, which have excitation energies less than 10³ K and consequently emissivities that are insensitive to electron temperature and temperature fluctuations, yield ionic abundances comparable to those derived from optical/UV CELs. For a given nebula, the discrepancies between the ORL and CEL abundances are of similar magnitude for different elements. In other words, relative abundance ratios such as C/O, N/O and Ne/O deduced from the traditional method based on strong CELs are comparable to those yielded by ORLs, for a wide range of ORL to CEL oxygen abundance ratios, varying from near unity to over a factor of 20. We have also determined ORL abundances relative to hydrogen for the third-row element magnesium for 11 nebulae in our sample. In strong contrast to the cases for second-row elements, Mg abundances derived from the Mg II 3d-4f λ4481 ORL are nearly constant for all the PNe analysed so far and agree within the uncertainties with the solar photospheric value. In accordance with results from previous studies, the ORL to CEL abundance ratio is correlated with the difference between the electron temperatures derived from the [O III] forbiddenline ratio, on the one hand, and from the hydrogen recombination Balmer discontinuity, on the other. We find that the discrepancy between the ORL and CEL abundances is correlated with nebular absolute diameter, surface brightness, the electron density derived from [S II] CELs, and excitation class. The results confirm that the dichotomy of temperatures and heavy elemental abundances determined from the two types of emission line, which has been widely observed in PNe, is a strong function of nebular evolution, as first pointed out by Garnett and Dinerstein. Our analyses show that temperature fluctuations and/or density inhomogeneities are incapable of explaining the large discrepancies between the heavy elemental abundances and electron temperatures determined from the two types of emission line. Our analyses support the bi-abundance model of Liu et al., who have proposed that PNe contain another previously unseen component of ionized gas which, highly enriched in heavy elements, has an electron temperature of ..10³ K and emits strongly in recombination lines but not in CELs. Our determinations of low average emission temperatures from the observed line intensity ratios of He I and O II ORLs lend further support to this scenario. [ABSTRACT FROM AUTHOR]

We have obtained deep optical spectra of medium resolution for a sample of 12 Galactic planetary nebulae (PNe). Optical recombination lines (ORLs) from carbon, nitrogen and oxygen have been detected in 11 of them and neon ORLs in nine of them. All spectra were obtained by scanning a long slit across the nebular surface, yielding relative line intensities for the entire nebula that are suitable for comparison with integrated line fluxes measured in other wavelength regions using space-borne facilities, such as the Infrared Space Observatory (ISO) and the International Ultraviolet Explorer (IUE). For 11 PNe, ISO infrared spectra between 2.4 and 197 µm are available, most of them taken by ourselves, plus a Kuiper Airborne Observatory (KAO) infrared spectrum of NGC 6210. IUE ultraviolet (UV) spectra are available for all nebulae except one in our sample. The UV, optical and infrared spectra have been combined to study nebular thermal and density structures and to determine elemental abundances. We have determined UV to optical extinction curves towards these PNe by examining observed fluxes of H I and He II recombination lines, radio free-free continuum flux density, and UV to optical nebular continua. For 11 PNe in our sample, the derived optical reddening curves are found to be consistent with the standard Galactic extinction law for a total-toselective extinction ratio, R ≡ A(V)/EB-V = 3.1. However, the optical extinction curve towards Hu 1-2 yields R = 2.0. The UV extinction towards Hu 1-2 and NGC 6572 is also found to be much steeper than the standard Galactic reddening law. In contrast, the UV extinction curve along the sight lines towards NGC 6210 is found to be much shallower, although in the latter case the uncertainties involved are quite large. Electron temperatures and densities have been derived using a variety of diagnostic ratios of collisionally excited lines (CELs) in the UV, optical and infrared. The results show clear stratifications, both in temperature and density. Lines emitted by ions formed in regions of higher ionization degree yield higher temperatures than lines arising from regions of lower ionization degree, while densities deduced from ratios of infrared diagnostic CELs of low critical densities, such as the [O III] 88-µm/52-µm ratio, are systematically lower than those derived from UV and optical diagnostic lines, which in general have much higher critical densities than the infrared fine-structure lines. Electron temperatures have also been derived from the ratio of the nebular continuum Balmer discontinuity to H 11 for 11 PNe. For four of these, the Balmer jump temperatures are more than 1000 K lower than values derived from the [O III] optical collisionally excited diagnostic line ratio. With a difference of 3580 K, NGC 40 has the lowest Balmer jump temperature relative to the [O III] optical forbidden-line temperature. High-order Balmer line decrements have been used to determine electron densities. The results are consistent with values derived from forbidden-line density-diagnostics. [ABSTRACT FROM AUTHOR]

In our Paper I, we presented deep optical observations of the spectra of 12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe, carrying out an abundance analysis using the collisionally excited forbidden lines. Here, we analyse the relative intensities of faint optical recombination lines (ORLs) from ions of carbon, nitrogen and oxygen in order to derive the abundances of these ions relative to hydrogen. The relative intensities of four high-lC iirecombination lines with respect to the well-known 3d–4fλ4267 line are found to be in excellent agreement with the predictions of recombination theory, removing uncertainties about whether the high C2+ abundances derived from theλ4267 line could be due to non-recombination enhancements of its intensity.We define an abundance discrepancy factor (ADF) as the ratio of the abundance derived for a heavy element ion from its recombination lines to that derived for the same ion from its ultraviolet, optical or infrared collisionally excited lines (CELs). All of the PNe in our sample are found to have ADFs that exceed unity. Two of the PNe, NGC 2022 and LMC N66, have O2+ ADFs of 16 and 11, respectively, while the remaining 13 PNe have a mean O2+ ADF of 2.6, with the smallest value being 1.8.Garnett and Dinerstein found that for a sample of about 12 PNe the magnitude of the O2+ ADF was inversely correlated with the nebular Balmer line surface brightness. We have investigated this for a larger sample of 20 PNe, finding weak correlations with decreasing surface brightness for the ADFs of O2+ and C2+. The C2+ ADFs are well correlated with the absolute radii of the nebulae, although no correlation is present for the O2+ ADFs. We also find both the C2+ and O2+ ADFs to be strongly correlated with the magnitude of the difference between the nebular[O iii] and Balmer jump electron temperatures, corroborating a result of Liu et al. for the O2+ ADF.is found to be weakly correlated with decreasing nebular surface brightness and increasing absolute nebular radius.There is no dependence of the magnitude of the ADF upon the excitation energy of the ultraviolet, optical or infrared CEL transition used, indicating that classical nebular temperature fluctuations– i.e. in a chemically homogeneous medium– are not the cause of the observed abundance discrepancies. Instead, we conclude that the main cause of the discrepancy is enhanced ORL emission from cold ionized gas located in hydrogen-deficient clumps inside the main body of the nebulae, as first postulated by Liu et al. for the high-ADF PN, NGC 6153. We have developed a new electron temperature diagnostic, based upon the relative intensities of the O ii4f–3dλ4089 and 3p–3sλ4649 recombination transitions. For six out of eight PNe for which both transitions are detected, we derive O2+ ORL electron temperatures of≤300 K, very much less than the O2+ forbidden-line and H+ Balmer jump temperatures derived for the same nebulae. These results provide direct observational evidence for the presence of cold plasma regions within the nebulae, consistent with gas cooled largely by infrared fine-structure transitions; at such low temperatures, recombination transition intensities will be significantly enhanced due to their inverse power-law temperature dependence, while ultraviolet and optical CELs will be significantly suppressed. [ABSTRACT FROM AUTHOR]

ABSTRACT The study of photoionized environments is fundamental to many astrophysical problems. Up to the present most photoionization codes have numerically solved the equations of radiative transfer by making the extreme simplifying assumption of spherical symmetry. Unfortunately very few real astronomical nebulae satisfy this requirement. To remedy these shortcomings, a self-consistent, three-dimensional radiative transfer code has been developed using Monte Carlo techniques. The code, mocassin, is designed to build realistic models of photoionized nebulae having arbitrary geometry and density distributions, with both the stellar and diffuse radiation fields treated self-consistently. In addition, the code is capable of treating one or more exciting stars located at non-central locations. The gaseous region is approximated by a cuboidal Cartesian grid composed of numerous cells. The physical conditions within each grid cell are determined by solving the thermal equilibrium and ionization balance equations. This requires a knowledge of the local primary and secondary radiation fields, which are calculated self-consistently by locally simulating the individual processes of ionization and recombination. The structure and the computational methods used in the mocassin code are described in this paper. mocassin has been benchmarked against established one-dimensional spherically symmetric codes for a number of standard cases, as defined by the Lexington/Meudon photoionization workshops: at Meudon in 1985 and at Lexington in 1995 and 2000. The results obtained for the benchmark cases are satisfactory and are presented in this paper. A performance analysis has also been carried out and is discussed here. [ABSTRACT FROM AUTHOR]

The rather rare class of central stars of planetary nebulae that show very low-excitation Wolf–Rayet spectra has been a subject of great interest, particularly in the infrared, since its discovery in the late 1960s. Further peculiarities have been found with the advent of infrared spectroscopy from ISO. Notably, these objects simultaneously betray the presence of regions of carbon-rich and oxygen-rich dust chemistry. We compare and contrast complete ISO spectra between 2 and 200 μm of a sample of six [WC8] to [WC11] central stars, finding many similarities. Among this sample, one star provides strong evidence of quasi-periodic light variations, suggestive of a dust cloud orbiting in a plane from which we view the system. [ABSTRACT FROM AUTHOR]

A correction to the article "New effective recombination coefficients for nebular N II lines," that was published in a previous issue of the jorunal is presented.

We identify emission lines of post-iron peak elements in very high signal-to-noise spectra of a sample of planetary nebulae. Analysis of lines from ions of Kr and Xe reveals enhancements in most of the PNe, in agreement with theories of $s$-process in AGB stars. Surprisingly, we did not detect lines from Br even though $s$-process calculations indicate that it should be produced with Kr at detectable levels. [ABSTRACT FROM PUBLISHER]